SQL> alter system quiesce restricted;
System altered.
- Only current command completed.
- No new connections allowed.
-New queries or attempted logins will appear to hang until you unquiesce
the database.
- select Active_State from V$INSTANCE; --This will tell you the status.
The Active_State
NORMAL (unquiesced),
QUIESCING (active non-
DBA sessions are still running),
QUIESCED

The suspend command suspends the database, and is not specific to an instance. Therefore, in an Oracle Real Application Clusters environment, if the suspend command is entered on one system, then internal locking mechanisms will propagate the halt request across instances, thereby quiescing all active instances in a given cluster. However, do not start a new instance while you suspend another instance, since the new instance will not be suspended.

In both cases, Sys and System are allowed to connect. All other users will be in hung state if they try to connect.

Enjoy.....and Hope this will save time as no shutdowns required for Maintainance (Small example may be, A data fix )